Air-hardenable bainitic steel with enhanced material characteristics

ABSTRACT

A method of producing a forged steel part is disclosed to include providing a steel billet having a composition including 0.25-0.40 wt. % C, 1.50-3.00 wt. % Mn, 0.30-2.00 wt. % Si, 0.00-0.150 wt. % V, 0.02-0.06 wt. % Ti, 0.010-0.04 wt. % S, 0.0050-0.0150 wt. % N, 0.00-1.00 wt. % Cr, 0.00-0.30 wt. % Mo, 0.00-0.003 wt. % B, and a balance of Fe and incidental impurities. The method may further include heating the steel billet to an austenization temperature of approximately 1150 degrees C. to 1350 degrees C., hot forging the steel billet to form the steel part, and controlled air cooling the forged steel part after the hot forging. The method may still further include induction heating select portions of the forged steel part after the controlled air cooling to increase the hardness of the select portions of the forged steel part, followed by quenching and tempering before the final machining.

This is a division of application Ser. No. 13/848,829, filed Mar. 22,2013 which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to an air-hardenable bainiticsteel and, more particularly, to an air-hardenable bainitic steel withenhanced material characteristics.

BACKGROUND

Structural components for machinery, such as track links used on theundercarriage of track-type earth moving machines, are required to havematerial characteristics that include good yield strength, good wearresistance, good toughness, and good rolling contact fatigue resistance.Track links used on the tracks of a track-type machine such as abulldozer or other earth moving equipment are well known in theindustry. A track link typically has a lower portion, or the body of thelink, and an upper portion, or the rail portion of the link. It isimportant that the rail portion of the track link have high surfacehardness, whereas the body portion of the track link can have lowersurface hardness for increased machinability. A high surface hardness inthe rail portion is necessary because the rail portion is subjected tosevere wear and spallation from continuous contact with track rollers. Alower surface hardness in the body portion of the track link allowsholes for bushings to be more easily machined into the body portion. Thelower surface hardness of the body portion of the track link also allowsfor a press fit between a bushing and the hole in the track link bodywithout creating excessive residual stresses.

The manufacturing processes for obtaining the desired materialcharacteristics in a track link or other structural component havetypically included hot forging the component from a steel billet,followed by cooling, reheating to austenization temperatures, quenching,and tempering. These heat treatment processes may then be followed byadditional heating of at least select portions of the component,quenching again, and tempering again before the final machining.Processing of a track link includes first heating a steel part toapproximately 1150-1350 degrees C. to bring the material to an austenitephase field, and then hot forging the part. The part is then slowlycooled to room temperature, followed by two heat treatment processes. Inthe first heat treatment process the track link is reheated toaustenization temperature, quenched to room temperature, and thentempered to a hardness of approximately 30-39 Rockwell C hardness (HRC).In the second heat treatment process just the rail portion of the tracklink is locally reheated by induction, quenched to room temperature, andtempered to a hardness of 51-57 HRC. These heat treatment processesresult in the track link having a hard rail and a softer body. The bodyof the link is then machined into final shape. The heat treatmentprocesses add significantly to the expense of producing the components,and also require significant capital expenditures for furnaces, as wellas ongoing maintenance expenses.

One attempt to produce alternative types of steel having good wear androlling contact fatigue resistance coupled with improved levels ofductility toughness and weldability is described in U.S. Pat. No.5,879,474 to Bhadeshia (“the '474 patent”) that issued on Mar. 9, 1999.The '474 patent discloses a steel for use in making a steel rail, wherethe steel is purported to provide a high strength, wear and rollingcontact fatigue resistant microstructure comprising carbide free“bainite” with some high carbon martensite and retained austenite in thehead of the rail.

Although the alloy steel disclosed in the '474 patent may provideimproved wear and rolling contact fatigue resistance, still furtherimprovements in manufacturing costs and material characteristics may bepossible. In particular, the '474 patent describes the use ofsignificant quantities of expensive alloying elements such as Chromium(Cr) and Molybdenum (Mo) to achieve improved levels of rolling contactfatigue strength, ductility, bending fatigue life and fracturetoughness, coupled with rolling contact wear resistance similar to orbetter than those of the current heat treated pearlitic rails.

The bainitic microalloyed steel produced in accordance with thechemistry and processes of the present disclosure solves one or more ofthe problems set forth above and/or other problems in the art.

SUMMARY

In one aspect, the present disclosure is directed to a method ofproducing a forged steel part, including providing steel billet having acomposition including, on a weight basis:

C: 0.25-0.40 wt. %,

Mn: 1.50-3.00 wt. %,

Si: 0.30-2.00 wt. %

V: 0.00-0.15 wt. %,

Ti: 0.02-0.06 wt. %,

S: 0.010-0.04 wt. %,

N: 0.0050-0.0150 wt. %,

Cr: 0.00-1.00 wt. %,

Mo: 0.00-0.30 wt. %,

B: 0.00-0.005 wt. %, and

a balance of Fe and incidental impurities, heating the steel billet toan austenization temperature of approximately 1150 degrees C. to 1350degrees C., hot forging the steel billet to form the steel part, andcontrolled air cooling the forged steel part after the hot forging.

In another aspect, the present disclosure is directed to anair-hardenable bainitic steel part having a composition including, on aweight basis:

C: 0.25-0.40 wt. %,

Mn: 1.50-3.00 wt. %,

Si: 0.30-2.00 wt. %

V: 0.00-0.15 wt. %,

Ti: 0.02-0.06 wt. %,

S: 0.010-0.04 wt. %,

N: 0.0050-0.0150 wt. %,

Cr: 0.00-1.00 wt. %,

Mo: 0.00-0.30 wt. %,

B: 0.00-0.003 wt. %,

a balance of Fe and incidental impurities, and a microstructure that isgreater than 50% by volume bainitic microstructure throughout the entiresteel part.

In yet another aspect, the present disclosure is directed to a forgedsteel part manufactured to have a chemical composition including, on aweight basis:

C: 0.25-0.40 wt. %,

Mn: 1.50-3.00 wt. %,

Si: 0.30-2.00 wt. %

V: 0.00-0.15 wt. %,

Ti: 0.02-0.06 wt. %,

S: 0.010-0.04 wt. %,

N: 0.0050-0.0150 wt. %,

Cr: 0.00-1.00 wt. %,

Mo: 0.00-0.30 wt. %,

B: 0.00-0.003 wt. %,

a balance of Fe and incidental impurities, a microstructure that isgreater than 50% by volume bainitic microstructure throughout the entiresteel part, and the forged steel part being manufactured by hot forging,controlled air cooling after the hot forging to produce a bainiticmicrostructure of greater than 50% bainite throughout the forged steelpart, and final machining.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary disclosed processeliminating typical heat treating steps;

FIG. 2 is a continuous cooling transformation (CCT) diagram forproducing the microstructure of an exemplary embodiment of thedisclosure; and

FIG. 3 is a flowchart depicting an exemplary disclosed method that maybe used to produce an exemplary microalloyed bainitic steel withenhanced characteristics.

DETAILED DESCRIPTION

A microalloyed, air-hardenable, predominantly bainitic steel withenhanced strength, wear, and toughness characteristics is disclosed. Themicroalloyed, bainitic steel may be economically produced withoutrequiring many of the heat treatment processes previously thoughtnecessary to achieve desired strength, wear, and toughnesscharacteristics. As shown in FIG. 1, the typical heat treatmentprocesses following hot forging of a steel part may include cooling,reheating to austenization temperature, quenching, and tempering. Theseheat treatment processes may then be followed by reheating selectportions of the steel part a second time using induction, quenching asecond time, and tempering a second time before the final machining. Atleast the first sequence of heat treatment steps involving reheating,quenching, and tempering may be required with conventional hot forgingprocesses in order to obtain desired strength and toughnesscharacteristics, while at the same time ending up with a part that isnot too hard for machining. Significant cost savings may be achieved ifat least some of these intermediate heat treatment processes can beeliminated. Capital investments for heat treatment capacity, andmaintenance costs on the furnaces and other equipment may be reduced. Incertain implementations of this disclosure the microalloyed, bainiticsteel may be provided with the necessary composition and cooling afterhot forging to arrive at a hardness of approximately 50-55 Rockwell Chardness (HRC) without even requiring the induction reheating,quenching, and tempering before final machining.

A predominantly bainitic microstructure according to variousimplementations of this disclosure is a microstructure that consists ofat least 50% by volume of a bainitic microstructure. Certain embodimentsmay have at least 70% by volume of a bainitic microstructure. Otherembodiments may have at least 85% by volume of a bainiticmicrostructure. Bainite is a microstructure that forms in steels attemperatures of approximately 250-550° C. (depending on alloy content).Bainite is one of the decomposition products that may form whenaustenite (the face centered cubic crystal structure of iron) is cooledpast a critical temperature of 727° C. (1340° F.) depending on alloycontent. A bainitic microstructure may be similar in appearance andhardness characteristics to tempered martensite.

A fine, non-lamellar structure, bainite commonly consists of cementiteand dislocation-rich ferrite. The high concentration of dislocations inthe ferrite present in bainite makes this ferrite harder than itnormally would be. As shown in the continuous cooling transformation(CCT) diagram of FIG. 2, the temperature range for transformation tobainite (250-550° C.) is between those for pearlite and martensite. Whenformed during continuous cooling, the cooling rate to form bainite ismore rapid than that required to form pearlite, but less rapid than thatrequired to form martensite (in steels of the same composition). Inaccordance with various implementations of this disclosure, amicroalloyed steel having the chemistry discussed in more detail belowmay be initially heated to austenization temperatures of approximately1150-1350° C. or greater. The steel may then be hot forged into thedesired shape, and control cooled from the forging temperature toachieve a bainitic microstructure. For the cooling after hot forging,atmospheric cooling or forced air cooling using a blower may beconducted. In various alternative implementations, the steel may becooled rapidly down to about the eutectoid transformation temperature,and then cooled slowly over a range from about 900 to 500° C. In stillfurther alternative implementations, the steel may be cooled quickly toabout 500 to 300° C. after hot forging, and may be kept at anequilibrium temperature somewhere in the range from about 500 to 300° C.to promote bainite transformation.

The cooling rate may be determined by reference to a CCT diagram, toknow the range for cooling rates passing through the bainitetransformation region and, thereby, controlling to the determinedcooling rate range. The CCT diagram may have been previously prepared,stored in a database, or otherwise made available for control of thecooling process. The forged product may be air cooled using fans orother means of circulating the cooling air to achieve a cooling ratethat falls approximately within the range from 0.5 to 5° C. per second,or 30 to 300° C. per minute, when cooling between approximately 900° C.and 500° C. Most alloying elements will lower the temperature requiredfor the maximum rate of formation of bainite, though carbon is the mosteffective in doing so. Bainite generally has a hardness that is greaterthan the typical hardness of pearlite and less than the hardness ofmartensite. Pearlite in the microstructure may contribute to reducedtoughness. The composition and processing of the microalloyed steelaccording to various embodiments of this disclosure are selected toavoid or at least minimize the amount of pearlite present. In commercialpractice a small amount of pearlite, such as less than 2 percent byvolume, may unavoidably be present, particularly in the center of largesections, but care is taken to minimize its presence and effects.

The bainite microstructure essentially has a two-phase microstructurecomposed of ferrite and iron carbide or cementite. Depending on thecomposition of the austenite during the hot forging process, and thecooling rate after hot forging, there is a variation in the morphologyof the resulting bainite. The resulting microstructures are referred toas upper bainite or lower bainite. Upper bainite can be described asaggregates of ferrite laths that usually are found in parallel groups toform plate-shaped regions. The carbide phase associated with upperbainite is precipitated at the interlath regions, and depending on thecarbon content, these carbides can form nearly complete carbide filmsbetween the lath boundaries. Lower bainite also consists of an aggregateof ferrite and carbides. The carbides precipitate inside of the ferriteplates. The carbide precipitates are on a very fine scale and in generalhave the shape of rods or blades. For this reason, the bainiticmicrostructure becomes useful in that no additional heat treatments arerequired after initial cooling to achieve a hardness value between thatof pearlitic and martensitic steels. The material characteristics of themicroalloyed and forged steel can vary over a large range depending onthe particular types and quantities of alloying elements included in thecomposition. When steel contains sufficient amounts of Si and/or Al, thecarbide formation can be significantly retarded such that carbide doesnot have enough time to form during the continuous cooling process,resulting in a mixed microstructure of bainitic ferrite and retainedaustenite. This type of bainitic microstructure may be referred to as“carbide free” bainite. It has been found that such bainite may providesuperior toughness to conventional types of bainite. The composition ofalloying elements included in accordance with various embodiments ofthis disclosure results in a steel part having the strength, hardness,and toughness characteristics previously only achieved by including theintermediate heat treatment steps following hot forging of reheating toaustenization temperature, quenching, and tempering.

The advantageous material characteristics discussed above are found tobe achieved to a greater extent as the percentage by volume of bainiticmicrostructure is increased. Accordingly, a part that is 70% by volumebainitic microstructure may exhibit greater strength, hardness, andtoughness characteristics than a part that is 50% by volume bainiticmicrostructure balanced with ferrite and/or pearlite type ofmicrostructure. Additionally, a part that is 85% or greater by volumebainitic microstructure may exhibit even further enhancedcharacteristics of strength, hardness, and toughness than the part thatis 70% by volume bainitic microstructure balanced with ferrite and/orpearlite type of microstructure. As shown in FIG. 1, intermediate heattreatment steps of reheating to austenization temperature, quenching,and tempering may be eliminated before the final machining of a forgedproduct in accordance with various implementations of this disclosure.Induction reheating of select portions of the steel part, such as therail portion of a track link used in contact with the tracks on earthmoving machinery, may be included to achieve enhanced hardness andstrength characteristics for certain parts or portions of parts. Theincreased hardness can also improve the wear resistance of the selectedportions of the steel part. The alloying elements that are added to thecomposition in accordance with various embodiments of this disclosuremay also be selected to obtain the desired volume percentages ofbainitic microstructure throughout the part, regardless of the differentcooling rates that may be experienced in different sections or portionsof the part having different thicknesses.

It has been discovered in various implementations of this disclosurethat the bainitic microstructure obtained after controlled air-coolingmay also exhibit the same or similar hardness and strengthcharacteristics as were previously achieved by following hot forgingwith quenching, reheating, quenching again, and tempering. Amicroalloyed steel may exhibit a martensitic microstructure after rapidcooling from hot forging temperatures through quenching in oil or water.The martensitic microstructure may have a Rockwell C hardness (HRC) of50 after quenching depending on the carbon content of the steel. Typicalmethods of processing this martensitic microstructure steel may theninclude reheating back up to austenitic temperatures of approximately800° C.-950° C., quenching again, and then tempering by reheating againto approximately 500° C.-590° C. in order to soften the steel toapproximately HRC 30. The controlled air cooling process for producing apredominantly bainitic microstructure according to variousimplementations of this disclosure may result in the same hardness ofHRC 30 without all of the quenching, reheating, quenching and temperingsteps previously required. As mentioned above, the predominantlybainitic microstructure may contain greater than 50% by volume ofbainitic microstructure. The hardness after air cooling in accordancewith this disclosure may fall within the range from approximately 35-45HRC. The types and quantities of microalloying elements included in thecomposition of the air-hardenable, bainitic steel in accordance withvarious embodiments may also enable achievement of hardness levels afterair cooling that fall approximately in the range from 40-55 HRC.

The microalloyed steel according to various implementations of thisdisclosure may have a chemical composition, by weight, as listed inTable 1:

TABLE 1 Composition of microalloyed steel in weight percent.Constituents Concentration by weight (%) Carbon 0.25-0.40 Manganese1.50-3.00 Titanium 0.02-0.06 Vanadium 0.00-0.15 Silicon 0.30-2.00Nitrogen 0.0050-0.0150 Optional Sulfur 0.010-0.040 Optional Chromium0.00-1.00 Optional Molybdenum 0.00-0.30 Optional Boron  0.00-0.005 Ironand other residual elements Balance

Carbon (C) contributes to the attainable hardness level as well as thedepth of hardening. In accordance with various implementations of thisdisclosure, the carbon content is at least 0.25% by weight to maintainadequate core hardness after tempering and is no more than about 0.40%by weight to assure resistance to quench cracking and steel toughness.It has been found that if the carbon content is more than about 0.40% byweight, water quenching may cause cracking or distortion incomplex-shaped articles and, in such cases, a less drastic quench mediumsuch as oil may be required. Therefore an advantageous range of C isfrom approximately 0.25-0.40 wt. %. The microalloyed, bainitic steelaccording to various implementations of this disclosure may be aircooled in accordance with select cooling curves on the CCT diagram ofFIG. 2.

Manganese (Mn) is a low cost allow and contributes to the deephardenability and is therefore present in most hardenable alloy steelgrades. The disclosed alloy steel contains manganese in an amount of atleast 1.50% by weight to assure adequate core hardness and contains nomore than about 3.00% to prevent manganese segregation and the formationof blocky retained austenite.

Silicon (Si) in amounts between approximately 0.30-2.00 wt. %, alongwith the Mn, allows the steel according to this disclosure to form apredominantly bainitic microstructure following air cooling from hotforging temperatures. The Si may also help deoxidation of the moltensteel, as well as contributing to the formation of a carbide-freebainite with improved toughness when sufficient Si is added into thesteel.

Chromium contributes to the hardenability of the present steel alloy andmay be added in small amounts not exceeding 1.00% by weight in order toallow for adjustment of the CCT curve to form a predominantly bainiticmicrostructure after air cooling. More Chromium increases steel cost.

Small amounts of other elements including Molybdenum (Mo) and Boron (B)may also be added to allow for further adjustment of the CCT curve toform a predominantly bainitic microstructure after air cooling.

Vanadium (V) and Nitrogen (N), despite their small quantities, may alsobe important ingredients in the present alloy steel composition, and maybe added to provide precipitation hardening and to realize aconsistently measurable enhancement of case and core hardness. Nitrogencombines with the titanium in the steel to form titanium carbonitridesto prevent grain coarsening during the reheat prior to forging andduring the cooling after hot forging. Without Ti and N, the forged steelmay have large prior austenite grain sizes, resulting in poor toughness.

The remainder of the alloy steel composition is essentially iron, exceptfor nonessential or residual amounts of elements which may be present insmall amounts. Titanium (Ti) may also be provided in amountsapproximately between 0.02-0.06% to form titanium carbonitrides toprevent grain coarsening before and after forging. Sulphur (S), which insmall amounts may be beneficial in that it promotes machining, may alsobe provided in small enough amounts so as to not contribute to any lossof ductility and toughness. Phosphorus (P) in an amount over 0.05% maycause embrittlement, and preferably the upper limit should not exceed0.035%. Other elements generally regarded as incidental impurities maybe present within commercially recognized allowable amounts.

Manufactured articles, such as track links, having the above statedcomposition, may be advantageously initially formed to a desired shapeby hot forging after heating up the microalloyed steel to austenizationtemperatures of approximately 1150-1350° C. The formed articles are thencontrolled cooled as described above to produce a predominantly bainiticmicrostructure. Select portions of the hot forged articles, such as therail portions of a track link, may then receive additional heattreatment by induction heating the select portions, quenching, andtempering before final machining to a desired final dimension.

FIG. 3 illustrates an exemplary method that may be used to produce apredominantly bainitic microalloyed steel part in accordance withvarious implementations of this disclosure. FIG. 3 will be discussed inmore detail in the following section to further illustrate the disclosedconcepts.

INDUSTRIAL APPLICABILITY

The steel, and method of making the steel in accordance with variousimplementations of the present disclosure may reduce costs byeliminating heat treatment steps typically performed after hot forging.The disclosed microalloyed, forged, air-hardenable bainitic steel mayprovide similar hardness, strength, and toughness characteristics topreviously hot forged and heat treated steel parts, but withoutrequiring all of the heat treatment processes. The microalloyingelements in combination with the controlled air cooling may produce apredominantly bainitic microstructure after air cooling from hot forgingtemperatures. Select portions of the steel parts produced in accordancewith the composition and processes of this disclosure may be furtherhardened if desired using localized induction heating followed byquenching and tempering. Alternatively, the composition of the bainiticsteel in accordance with this disclosure may be adjusted within thedisclosed ranges, and air cooled in order to achieve a hardness afterair cooling falling in the range from approximately 50-55 HRC with nofurther heat treatment.

The steel part produced in accordance with various advantageousimplementations of this disclosure exhibits material characteristicsthat may include a body hardness of 35-45 HRC after air cooling for goodmachinability with no heat treatment, a body yield strength that isgreater than 1000 mega-pascals (MPa) after the air cooling, a hardnessat the portions that have been additionally hardened through selectiveinduction heating of greater than approximately 50 HRC, and a bodytoughness of approximately 20 Joules or greater in the Charpy impacttest at room temperature.

As shown in FIG. 3, at step 320 a microalloyed steel having thecomposition shown above in Table 1 may be heated to austenizationtemperatures of approximately 1150 degrees C. to 1350 degrees C. Thetypes of parts being manufactured in accordance with variousimplementations of this disclosure may include parts that require goodmachinability in at least one portion, high yield strength, good wearcharacteristics, and good toughness. One exemplary application of thedisclosed compositions and processes is for track links used in thetracks of a track-type machine such as a bulldozer or other earth movingequipment. The size of the parts determines the size of a steel billetthat is initially heated to austenization temperatures in accordancewith step 320.

At step 322, the heated billet may be hot forged to a desiredconfiguration. After the hot forging, step 324 may include air coolingthe hot forged product at a cooling rate that results in the formationof a predominantly bainitic microstructure throughout the hot forgedpart. As shown by the CCT diagram of FIG. 2, the cooling rate may bechosen to avoid the formation of a large amount of martensiticmicrostructure or a predominantly ferrite and pearlite microstructure.In various implementations of this disclosure, the hot forged steel maybe cooled at a rate that falls approximately in the range from 0.5 to 5degrees C. per second as the steel cools from approximately 900 degreesC. to approximately 500 degrees C. In various alternativeimplementations, the weight percentages of the alloying elements in thecomposition of the steel may be varied in order to change the phasetransformation curves on the CCT diagram, and achieve the desiredpredominantly bainitic microstructure at cooling rates achieved bytransporting the hot forged steel parts along a conveyor at ambienttemperatures. The microalloyed steel may also be advantageously providedwith a composition that achieves the desired bainitic microstructure andhardness levels throughout the part even at the different cooling ratesthat may be experienced by different sections of the part havingdifferent thicknesses. The predominantly bainitic microstructure may bea microstructure with greater than 50% bainite, or more advantageouslygreater than 70% bainite, or still more advantageously greater than 85%bainite throughout the hot forged steel part. The hardness levelsthroughout the entire forged steel part after air cooling may fallwithin the range from approximately 35-45 HRC. In other advantageousembodiments, the hardness levels throughout the forged steel part mayfall within the range from approximately 40-55 HRC after air coolingwith no further heat treatment.

At step 326, select portions of the steel part may be induction heatedin order to achieve higher hardness levels. In the exemplaryimplementation of a track link, a high surface hardness may be desiredfor the rail portion because the rail portion may be subjected to severewear from continuous contact with track rollers. A lower surfacehardness in the body portion of the track link allows holes forbushings, pins, and bolts to be more easily machined into the bodyportion. The lower surface hardness of the body portion of the tracklink also allows for a press fit between a bushing and the hole in thetrack link body without creating excessive residual stresses. In variousexemplary embodiments, the hardness of the induction heated portions ofthe steel part may fall within the range from approximately 50-57 HRC.

At step 328, after select portions of the steel part have been inductionheated, at least these heated portions of the steel part may bequenched, using techniques such as directed sprays of quenching fluidsonto the induction heated areas of the part. Following quenching, atstep 330, the steel part may be reheated to tempering temperatures inorder to improve its toughness. Final machining may then occur at step332.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed microalloyedsteel and method of forming the steel into a finished part withoutdeparting from the scope of the disclosure. Alternative implementationswill be apparent to those skilled in the art from consideration of thespecification and practice disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope of the disclosure being indicated by the following claims andtheir equivalents.

What is claimed is:
 1. A method of producing a forged steel part,comprising: providing steel billet having a composition comprising, on aweight basis: C: 0.25-0.40 wt. %, Mn: 1.50-3.00 wt. %, Si: 0.30-2.00 wt.% V: 0.00-0.15 wt. %, Ti: 0.02-0.06 wt. %, S: 0.015-0.04 wt. %, N:0.0050-0.0150 wt. %, Cr: 0.00-1.00 wt. %, Mo: 0.00-0.30 wt. %, B:0.00-0.005 wt. %, and a balance of Fe and incidental impurities; heatingthe steel billet to an austenization temperature of approximately 1150degrees C. to 1350 degrees C.; hot forging the steel billet to form thesteel part; and controlled air cooling the forged steel part after thehot forging, wherein the controlled air cooling is performed at a rateto produce a body hardness for the steel part of approximately 35-45Rockwell C hardness (HRC) after the controlled air cooling, a yieldstrength for the steel part of approximately greater than 1000mega-pascals (MPa) after the controlled air cooling, and wherein atoughness of at least an inner portion of a body of the steel part afterthe controlled air cooling is approximately greater than or equal to 20Joules at room temperature in accordance with the Charpy impact test. 2.The method according to claim 1, wherein the controlled air cooling isperformed by moving the forged steel part along a conveyor at ambienttemperatures.
 3. The method according to claim 1, wherein thecomposition of the steel billet is selected such that the controlled aircooling of the forged steel part and resultant different rates ofcooling of sections of the forged steel part having differentthicknesses results in a microstructure throughout the entire forgedsteel part after the controlled air cooling of approximately greaterthan 50% by volume of bainitic microstructure.
 4. The method accordingto claim 1, wherein the composition of the steel billet is selected suchthat the controlled air cooling of the forged steel part and resultantdifferent rates of cooling of sections of the forged steel part havingdifferent thicknesses results in a hardness level throughout the entireforged steel part after the controlled air cooling of approximatelygreater than 35-45 HRC.
 5. The method according to claim 1, furtherincluding induction heating select portions of the forged steel partafter the controlled air cooling to increase hardness of the selectportions of the forged steel part.
 6. The method according to claim 5,wherein the hardness of the select portions of the forged steel partafter induction heating is greater than approximately 50 HRC.
 7. Themethod according to claim 6, further including quenching at least theinduction heated portions of the forged steel part and reheating totemper the select portions of the forged steel part for enhancedtoughness.
 8. The method according to claim 1, wherein the compositionof the steel billet and the controlled air cooling of the forged steelpart result in a microstructure of the forged steel part after thecontrolled air cooling of approximately greater than 85% by volume ofbainitic microstructure.
 9. The method according to claim 1, wherein thecomposition of the steel billet and the controlled air cooling of theforged steel part result in a microstructure of the forged steel partafter the controlled air cooling of approximately greater than 70% byvolume of bainitic microstructure.